IMO, 18s will look kinda goofy at stock ride height, I'd go 17s. Also recommend going same size all around as mentioned above. Low-powered car with front-biased weight distribution doesn't really want wider rear wheels/tires. I'd go 8" or 8.5" all around.
I'd make sure the tires are 25" or smaller in diameter (255/40-18 are 26"). Taller tires will hurt acceleration and overall performance.
I'd be looking at 8" - 9" wheels with 225/45-17 (225/40-18), 235/40-17 (235/35-18), or 245/40-17 (245/35-18).
The Tenjins are kinda heavy, too, which will impact performance a bit. If you really really want 18s and like the 9-spoke black w/ machined lip style, I'd get the O.Z. Allegerita 18x8:
http://www.tirerack.com/upgrade_gara...14&showRear=no
Only $60 more per wheel vs. Tenjin, and 4 lb. lighter at 17.5 vs. 21.5.
Personally not a fan of black-painted wheels, though the machined lip does give some relief...